1. Field of the Invention
The present invention relates to prepress color proofing methods and materials which can be used for generating images on any available printing stock.
2. Background of the Art
In printing pictorial matter, whether by lithography, letterpress or gravure, the halftone process is used, wherein the actual printing image is composed of thousands of minute dots per square inch of a single color ink of varied dot size or ink density. What the naked eye sees as shading in halftone prints as actually a controlled variation in the size of dots relative to the unprinted areas between the dots. In black and white pictorial matter the dots are printed in black ink only. Full color reproductions, however, are necessarily printed in each of at least three colors, cyan, magenta, and yellow (known as "three color process"), or in these same colors with the addition of black ("four color process"). For each color a printing plate is made. In order to make the three or four printing plates, the original color picture or photograph is "separated" photographically, with the use of filters, masks, etc., into a set of three or four halftone negatives, each representing one of the colors, and containing, dot for dot, the amount of that color which must be printed to produce the desired total color print.
The preparation of the color-separation negative is an art and requires considerable skill in handling the many variables to produce a desired result. Often trial and error is involved requiring correction or replacement of one or more of the negatives. Unless some reliable system is available to "proof" the negatives, the printing press must be set up and copy printed just to secure preliminary proofs. This is time consuming and expensive. It is therefore desirable to provide accurate means for proofing the negatives without printing.
One system for proofing color separation negatives is disclosed in U.S. Pat. No. 3,136,637. A light-sensitive transparent sheet is provided for each of the colors to be printed. Each of the sheets is exposed through its respective color separation negative. Upon processing, the color in the non-image areas is removed, yielding a sheet which contains the desired color pattern in the image areas, while being colorless and transparent in the non-image areas (e.g. between halftone dots). After each of the separate sheets is made, they are assembled together in registry on a white background, whereupon a color proof results.
That system of color proofing has a number of inherent drawbacks. For example, the laying up of the multiplicity of sheets requires that the viewer look through a plurality (three or four) of transparent films during the proofing operation. Since the composite is made of several separate sheets, extreme care is required to maintain registry. If the individual sheets are not perfectly colorless and transparent in the optical sense, any "haze" or imperfection present is multiplied in the several sheets. Additionally, incident light reflects from the several sheets imparting a gloss which is not truly representative of printed copy, thus introducing a need for interpretation in evaluating the proof.
U.S. Pat. No. 3,671,236 improves upon the proofing system described in U.S. Pat. No. 3,136,637. Photo-mechanically produced images corresponding with each color are integrally built up on a single substrate (much as occurs in the actual printing operation itself) without any printing operations. The multiplicity of carrier films is eliminated by transferring individual color images from a sheet comprised of 1) a carrier with release surface, 2) pigment and binder layer, 3) photohardenable or insolubilizable layer, 4) barrier layer and, 5) pressure-sensitive adhesive layer.
U.S. Pat. No. 4,656,114 provides a thermally transferable prepress proofing sheet by using thermal adhesives, particularly acrylic adhesives, that do not discolor during imaging of the sheet and is non-tacky at room temperature.
U.S. Pat. No. 4,497,889 describes compounds which can be added to negative acting photopolymerizable compositions to improve their release properties. The compositions can be resistively developed in aqueous or organic solvent systems. Testing procedures for the peel forces are described.
In the above mentioned art, special bases must be used for the following reasons: (1) Coated paper stock must be used to prevent the adsorption of the developer solution during processing. The developer would discolor and distort the paper preventing good registration of the color images. (2) Thick paper stock must be used to prevent wrinkling and distortion during lamination. (3) A highly reflective layer is present to give a standard whiteness (which is not desirable in all cases as will be seen). The use of this special base means that the final assembled proof does not match the printed page in visual color, texture, or feel as most printed jobs are not on heavy, white, plastic coated papers. It is highly desirable for the final proof to match the expected press printed copies as close as possible as it leaves less interpretation of the differences seen in the proof by the operator of the printing press.
U.S. Pat. No. 4,482,625 discloses a process for the preparation of a color proofing sheet in which a multicolor proofing image is formed and the image is placed in a photopolymerizable composition, the composition transferred, and the composition photopolymerized and hardened.
U.S. Pat. No. 4,686,163 discloses a process for transferring a multicolor electrostatically toned image onto a permanent receptor. The multicolor toned image is formed on a first support and is itself transferred to a second support.